1. Field of the Invention
The present invention relates to a process for producing selectively metallized articles, e.g., molded printed circuit boards and other selectively conductive molded articles. A unique etching procedure is provided in which the solvent swelling step used in conventional procedures is alternated with exposure to a caustic solution to improve adhesion of metal deposits to the articles.
2. Discussion of the Background
Circuit boards are widely used in the electrical industry for radio, television, computers, appliances, industrial and electronic equipment. Printed circuit boards have been traditionally manufactured from a copper clad epoxy-glass laminate.
When starting with this material the shape of the printed circuit board must first be routed out and the holes for mounting the components (e.g., transistors, resistors, integrated circuits, etc.) individually drilled. The board is then masked with photoresist, the circuitry imaged, and the copper etched away from areas where it is not wanted.
An alternative to this procedure is to injection mold the circuit board substrate with the holes in place or even with parts such as clips and bosses as an integral part of the molded article. The molded substrate is then put through several adhesion promotion steps and plated with electroless metal according to standard technology, to produce the printed circuit board. In this case the substrate material is limited to thermoplastic resins with sufficient thermal stability and chemical properties to survive wave soldering. Savings may result with these injection molded circuit board substrates due to the elimination of considerable mechanical processing such as routing and drilling.
The critical parameters of a printed circuit board, from a soldering standpoint, are its heat deflection temperature, environmental stress crack resistance and thermal expansion coefficient. The higher a substrate's heat deflection temperature and environmental stress crack resistance to solder fluxes, the less likely it will blister or delaminate during soldering.
Methods for the electrochemical and electroless deposition of copper and other conductive metals onto activated substrates are well known and defined in the art and will not be detailed herein. Methods for manufacturing planar printed circuit boards from metal/resin laminates by subtractive etching of the copper are also well documented in the art.
The image of the circuit trace can be obtained by various methods known in the art, including use of resists before or after initial metal plating, or by overmolding with a noncatalytic resin as disclosed in European Patent Application Nos. 0 256 428 and 0 192 233. In these cases, it is necessary to etch the material both to promote good adhesion and to expose the catalytic particles, which are generally remote from the original molded surface of the device.
The demand for nonconductive moldable materials that will accept electroless deposits of metal is clear. In common to all of the above techniques of electroless metal deposits is the need to etch, adhesion promote, or otherwise modify the surface of such a molded part.
Many techniques for chemical etching of thermoplastics are disclosed in the literature, including U.S. Pat. Nos. 4,457,951, 4,592,929, 4,595,451, 4,601,783, 4,610,895, 4,592,852 and 4,629,636. In general, these techniques require the material to be wet or penetrated by reagents or solvents.
Alternative methods, which utilize plasmas, as in U.S. Pat. Nos. 4,337,279 and 4,402,998 are more expensive and difficult to carry out, as well as exhibiting some difficulties with uniform etching and penetration into holes. For these reasons, chemical etching methods involving solvent penetration into the material are preferred.
Available materials are limited in scope by this need for chemical attack, which is usually dependent on penetration of a solvent into the material. This is best accomplished with an amorphous polymer, which typically has a low solvent resistance, as compared to a semicrystalline polymer. Thus, most common polymers used in the art for molded circuit boards, polyethersulfone (PES) and polyetherimide (PEI), are amorphous polymers.
There are many advantages to be gained by the use of semicrystalline polymers. A property of prime importance in the field of molded circuit boards is a high Heat Deflection Temperature (HDT), since in many cases it is desirable to solder electrical components onto these devices, generally at temperatures in the range of 230.degree. to 290.degree. C. HDT is measured via ASTM D648, however, tests for dimensional stability of the molded parts in these temperature ranges are the standard by which sufficiently "high HDT" is judged.
For an amorphous polymer this requires a very high glass transition temperature, Tg, which restricts the available polymers to an elite group. These polymers are typically expensive, require high processing temperatures, and have very poor flow characteristics.
Semicrystalline polymers, on the other hand, can be compounded with glass fibers or other fillers to give a HDT determined more by their Tm (melting temperature) rather than their Tg. Generally, at least about 10% glass fiber is needed. This results in much higher HTD's than would otherwise be indicated by their melt viscosities and processing temperatures of these polymers, and would allow for the use of much less expensive polymers with lower processing temperatures and good flow characteristics.
For example, poly(cyclohexylenedimethylene terephthalate) (PCT) can withstand total immersion in a 260.degree. C. solder bath for a full minute with little ill effect. In contract, PES, which has a similar processing temperature, a higher melt viscosity, and higher cost, has a Tg about 40 degrees below 260.degree. C.
Semicrystalline polymers clearly would be competitive in both cost and HDT, if they could be made to accept electroless metal plating with good adhesion.
Thus, there remains a need for a method for increasing the adhesion of deposits of electroless metal plating onto a molded article, particularly when the molded part comprises at least some semicrystalline polymers.